The ability to produce a composite material having specified properties is of great importance in a variety of military, as well as commercial fields. Being able to monitor the production of these materials in order to assure the presence of the essential properties required for a specific purpose would be a significant advancement in the composite material art. Monitoring the production of a resin material to make sure that it has certain, unique properties such as strength, stiffness and weight, or combinations of said properties, would enable material scientists to better develop the materials needed. Such monitoring would allow one skilled in the composite material art to successfully produce a material needed with less waste, and with the ability to identify any unfavorable properties of the composite material while it is in the production stage, as opposed to in the testing stage. It allows for variations in the production process to be made in the early stages of development.
The prior art teaches the use of fiber optics in detecting damage in composite structures. Hofer, in an article entitled "Fiber Optic Damage Detection in Composite Structures," Composites, Volume 18, No. 4 (September, 1987), describes a method for inspecting a composite material for remote damage. The method described utilizes surface mounted or embedded fiber optic cables. Similar to the present invention, the fiber optic cables can be drawn out to diameters comparable to that of the composite material reinforcement. Hence, the fiber optic cables can be embedded permanently in the composite without significantly influencing the overall integrity or properties of the composite material. Although the present invention may use the fiber optic cables described in the article, nowhere in the article is it taught that the fiber optic arrangement therein provides a means for determining resin location and cure during composite fabrication. Moreover, nowhere in the article is the grid-like system of the present invention set forth. The fiber optic system described in this article does not possess the novel features within the scope of the present invention.
A wide variety of devices already exist for monitoring the curing process of a wide range of resin materials. These existing devices utilize a variety of dielectric or microdielectric techniques to measure the electrical resistance or capacitance of the resin material being produced. The information obtained by these prior art techniques is then passed to a series of conventional signal conditioners, computer hardware and software. The conventional signal conditioners, computer hardware and software then interpret said information and displays it in a fashion which illustrates the curing process of the resin materials.
Prior art in composite monitoring utilizes the basic, conventional dielectric and microdielectric techniques. These techniques have been available, and have been used in the art for quite some time. Some patents illustrating the use of these well-known techniques are discussed below.
Senturia et al., U.S. Pat. No. 4,423,371, teaches a method for determining the moisture level in dielectric materials. The invention therein makes use of a well-known "comb" configuration in its monitoring apparatus. The "comb" configuration of Senturia et al. is illustrated in FIG. 2. The "comb" configuration enables the total dielectric sensing area to be increased. This configuration is used in the sensor device employed therein. Senturia et al., however, does not make use of the gap/grid concept employed by the present invention; and therefore, is unable to distinguish the location of varying moisture levels in said materials.
Day et al., U.S. Pat. No. 4,777,431, teaches an apparatus for monitoring dielectric changes in a wide variety of polymeric resin materials. The device within the scope of Day et al.'s invention is designed for sensing changes in the dielectric properties of materials undergoing state transitions such as resins undergoing curing. The invention includes the development of a semi-permeable microdielectric sensor for use with graphite or ion graphing reinforcement and PMR-15 resin systems. PMR-15 resin systems are high temperature/high performance resins. The apparatus invented by Day et al, is adapted in such a manner that it is to be at least partially implanted into the material which is to be monitored. The sensor electrodes used in the patent are planar porous or mesh materials (see column 3, lines 52+). The sensor developed by Day et al., however, does not employ or teach the gap/grid encompassed by the invention herein. Moreover, the invention herein does not make use of conventional dielectric or microdielectric sensor materials.
The sensors taught in the above patents cannot be incorporated into the composite materials to be monitored in great number because of the size, cost, and limited means of attachment. Since these sensors cannot be incorporated into the materials in large number, the uniformity of properties of the composite material being produced cannot be accurately monitored. Moreover, the prior art sensors, even if they could be incorporated into the composite materials in great number, are prohibitively expensive; therefore, an alternate monitoring apparatus and method must be considered.
Berry, U.S. Pat. No. 3,383,863, teaches a method of detecting leaks in a pond, tank and pit. Berry makes use of a grid of electrical wires wherein the resistance between different wires of the grid are measured. It appears that the grid of electrical wires in said reference need not intersect with one another as one would expect in a conventional grid configuration. Note column 2, lines 65-70. Although Berry appears to teach the specific grid-like configuration employed by the present invention, nowhere does the reference suggest employing said grid-like sensor to monitor resin flow or resin cure. Moreover, Berry makes use of an ohmmeter. Use of the monitoring apparatus of the present invention, i.e., a scanner and/or computer device, are not even remotely suggested by the teachings in Berry.
The present invention offers an apparatus and method that is convenient, because of its use of conventional, electrically conductive, sensor threads; inexpensive, because of its maximizing the use of each sensor thread; and reliable. The sensing grid of the present invention can be embedded not only in the composite reinforcement, but it can be part of the bleeder, breather, and bagging materials, as well. The invention provides a means for monitoring resin migration during autoclave, hot press and other prepreg composite processes. The present invention provides a means for evaluating and monitoring thorough wet-out and cure, as well as a means for determining optimum mold and process configurations of the composite resin materials as they are being produced. Furthermore, the device can provide three dimensional flow field information, as well as local resin flow front velocities.
The present invention makes use of electrically conductive threads in a set grid-like configuration. The configuration used herein is considerably different from any resin cure or resin flow sensor previously used. The configuration of the sensors allows for more efficient monitoring of the resins using a minimal number of sensors threads. The cost of performing the monitoring function is, therefore, kept to a minimum. To date, an instrument for monitoring the cure of a wide range of resin systems using sensors that are incorporated as integral components of the composite resin structure has not successfully been proposed or accomplished. Moreover, a sensor system using a grid-like array of non-intersecting electrically conductive threads to serve as leads that can be connected to an interpreting apparatus (i.e. a computer) has also not been proposed.
Although grid-like electrically conductive devices have been employed in various art areas, they are herein applied for the first time in the monitoring of resin flow and resin cure. In addition, said grid-like electrically conductive devices to date have not been coupled with conventional scanner devices or computers. The present invention does not reside in the individual components of the electrically conductive device, the scanner or the computer. The invention resides in the combination of said conventional devices and in the use of said combination.